This invention relates generally to devices for processing and analyzing high-frequency signals and, more particularly, to signal spectrum analyzers or channelizers. U.S. Pat. No. 4,541,687 issued to Brooks, discloses such a device employing surface acoustic waves and a surface acoustic wave substrate as the transmission medium.
A surface acoustic wave channelizer has an input transducer array to which a broadband input signal is applied. The input array provides, in effect, a number of point sources of acoustic wave, which, for a particular frequency, combine constructively at a predetermined point in a focal region on the substrate. Output transducers are placed on the focal region, and operate to convert the focused acoustic energy back into electrical signals at a number of different frequencies. The device operates as a spectrum analyzer or Fourier transform device, converting a single broadband input signal into multiple narrowband output signals indicative of the spectral content of the input signal.
Although the surface acoustic wave channelizer operates satisfactorily, if has a two significant limitations. First, like all surface acoustic wave devices, the surface acoustic wave channelizer operates at frequencies below 1 gigahertz (GHz). This is a limitation imposed by the velocity of propagation of the acoustic waves and by the minimum resolution obtainable in photolithographically forming the transducers of the device. Some applications require the analysis of signals at much higher frequencies than this, and the higher frequency input signal must first be down-converted to below 1 GHz before input to the channelizer. Down-converting adds complexity and cost to the analysis procedure, and it would be desirable to avoid this step if possible.
Another disadvantage of the surface acoustic wave channelizer is that surface acoustic wave substrates are anisotropic in nature. In particular, they exhibit different wave propagation velocities for different directions of propagation. These differences have to be compensated for by appropriate placement and orientation of the input transducers.
Accordingly, there is still a need for a channelizer capable of handling high frequencies, in excess of 1 GHz, directly and without the need for down-converting at the input of the device, and having a more isotropic transmission medium than the ones used in surface acoustic wave devices. The present invention satisfies this need.